I can't help with consumption figures but can tell you that long range tanks were made for the P38A by a company in Australia - Brown Davis. I had a 150 litre tank in mine. There are two variations, one for GEMS and one for the later Bosch Motronic cars.
This is the one I had in my GEMS.
P38A Long Range Tank 002.jpg
This how far they hang down:
P38A LR TAnk.JPG
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